Global positioning systems (GPS) are used in vehicles to determine the current position of the vehicle on the earth's surface. Current position data may be used in applications such as on-board navigation systems, remote navigation, emergency response and concierge service systems, fleet tracking applications, speed restriction violation monitoring for insurance loading purposes, vehicle recovery, and many others. One problem with global positioning systems is that they rely on an uninterrupted line-of-sight between the vehicle and a required minimum number of the satellites deployed in the particular system. In many areas or regions, particularly urban areas or regions, the line-of-sight between the vehicle and a sufficient number of satellites is not possible to maintain. Tunnels, underground or covered parking areas, and areas with many tall buildings are examples of such areas where the signal from the satellites to the global positioning system of the vehicle may be interrupted.
In order to correct for such gaps in satellite communications, global positioning systems are typically equipped with gyroscopic sensors, multi-axis accelerometers, differential compasses or magnetic compasses or the like. Data from these sensors, when combined with speed or distance traveled information from other sensors on the vehicle, may be used to determine a vehicle trajectory since the last point of satellite contact. This information is important in order to maintain the service for which the GPS data was intended, when no GPS data is available. Such information is also important to enable a rapid lock onto the available satellites when the vehicle re-emerges from the constrained situation, since the system may calculate which satellites should be in view from the new position and time and begin its search for those satellites first.
However, such gyroscopes and multi-axis accelerometers are costly and may provide inaccurate data at low speeds. Also, magnetic compasses are subject to error due to significant ferrous content in the structures common in those areas in which satellite views are restricted. Additionally, differential compasses may provide erroneous data due to differences in the rolling radii of the vehicle wheels.